1. Field of the Invention
The present invention relates to an image forming apparatus that transfers a toner image borne on an image bearing member onto a recording medium using an electrophotographic technology for a copying machine or a laser printer or the like. More particularly, the present invention relates to an image forming apparatus having a transfer belt that performs transfer of the toner image onto the recording medium, and conveyance of the recording medium.
2. Description of the Related Art
In an electrophotographic apparatus in which a recording medium is borne and conveyed by a transfer belt stretched by a plurality of rollers, a recording material on the transfer belt, when passing through a transfer nip portion, is electrostatically attracted to the transfer belt.
However, if a stiffness of a recording medium is weak, the recording medium cannot be separated from the transfer belt by only utilizing a curvature of a separation roller that stretches the transfer belt, and the stiffness of the recording medium. As a result, the recording medium remains attached to the transfer belt at a position of the separation roller, and thus an improper separation occurs.
Thus, as a configuration for providing undulations on a transfer belt at a separation position, there is a method for separating a recording medium by uniformly forming protruding objects on a surface of the separation roller that stretches the transfer belt, for example, as discussed in Japanese Patent Application Laid-Open No. 9-015987. Although undulations can be formed on the transfer belt at the separation position, by using such configuration, they will always cause a great tension to locally act on the transfer belt. As a result, transferability becomes unstable because of the effects of uneven resistance caused by occurrence of local wear of the transfer belt.
A method for reducing wear caused by deformation, even while causing a cylindrical-shaped transfer material bearing sheet that bears the recording medium to be deformed for separating the recording medium, is discussed in Japanese Patent Application Laid-Open No. 5-119636. In Japanese Patent Application Laid-Open No. 5-119636, there is discussed a configuration in which a roller is provided as a lifting means movable between a position at which a transfer sheet is lifted from an inner side and a position at which it is not lifted.
In a method discussed in Japanese Patent Application Laid-Open No. 5-119636, separation of the recording medium is performed by lifting the transfer sheet by using the roller, and the transfer sheet is not lifted as long as the recording medium is not separated. In order to separate recording media with different thicknesses without causing the transfer material sheet to be deformed more than necessary, a method for separating a thin recording medium by a large lifting amount, and separating a thick recording medium by a small lifting amount is discussed in Japanese Patent Application Laid-Open No. 5-341664.
By applying such configuration to the transfer belt, there is built a configuration for arranging a lifting means for performing an operation of locally lifting the transfer belt in a width direction of the transfer belt during separation step, on the downstream side in a recording medium conveying direction from a transfer member that transfers a toner image onto the recording medium on the transfer belt.
If a stiffness of a recording medium such as a thin paper sheet is weak, undulations can be formed on the recording medium by conveying the recording medium in a state where the transfer belt is locally lifted, and a stiffness of the recording medium during the separation step can be increased.
When a distance of protrusions formed on the transfer belt in a width direction is made larger, a region where it is hard to form undulations on the recording medium will be generated between adjacent protrusions. Thus, in order to restrain generation of the region where it is hard to form the undulations in the recording medium between adjacent protrusions, it is desirable to keep a distance between adjacent protrusions.
On the other hand, in order to make the stiffness of the recording medium greater, it is effective to make a lifting amount which the lifting means lifts the transfer belt greater, and to make protrusions to be formed on the transfer belt higher.
However, if the protrusions are made high, while keeping a distance between adjacent protrusions short, a valley shape formed between adjacent protrusions will become steeper and deeper. As a result, it becomes difficult for the recording medium to follow a bottom of the valley shape, and a distance between the recording medium and the transfer belt becomes large at the bottom, whereby there is a risk of deteriorating a toner image of the recording medium.